Computer disk drives store digital information on magnetic disks. The magnetic disks are generally coated with a magnetic material capable of changing the direction of its magnetic orientation in response to an applied magnetic field. Information is stored on the magnetic disks as a series of magnetic transitions. Typically, the information is stored on each disk in concentric tracks that are divided into servo sectors and data sectors. Information is written to or read from a disk by a transducer head mounted on an actuator arm that is capable of moving the transducer head radially over the disk. The movement of the actuator arm allows the transducer head to access different tracks. The disks are rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors within each track on the disk. The transducer head may include integrated read and write heads.
In response to the increasing need to store large amounts of digital data in connection with computer systems, magnetic storage devices have utilized increased data storage densities. In order to support high data densities, the magnetic material of the magnetic disks must be provided in a very thin layer. In addition, high data densities require magnetic material with a small grain size. A thin layer and a small grain size reduce noise, and allow magnetic transitions to be more closely spaced together. However, the energy required to switch the magnetization of the material is decreased when the magnetic material has a small grain size and is provided in a thin layer. Accordingly, as the grain size and the layer thickness of the magnetic material has decreased, the material has become more susceptible to data loss due to thermal decay.
Thermal decay is related to the ratio of the energy barrier that must be crossed in order to switch the magnetization of the magnetic material of a magnetic disk to the thermal energy of the surrounding environment. In general, as the energy in the environment becomes more nearly equal to this energy barrier, thermal decay is more likely to occur. A magnetic disk having only a thin layer of magnetic material is particularly susceptible to thermal decay because the energy required to switch the magnetization of a portion of that disk is low. In addition, when data is stored at high densities, the area of the disk used to store a bit of information as a particular magnetic polarity (i.e. a bit cell) is small. Therefore, the energy required to switch the magnetization of a bit cell is reduced with increased areal densities. Furthermore, as grain sizes have been reduced, the anisotropic energy associated with each grain has also been reduced. As the anisotropic energy of each grain becomes nearer to the ambient thermal energy in a disk drive, information stored on the magnetic disk is more likely to be lost due to thermal decay.
As will be understood by those skilled in the art, the anisotropic energy of a grain is the fixed amount of energy required to maintain a stored direction of magnetization in the magnetic material, and is equal to the anisotropic energy density, Ku, times the volume of the grain, V. A thermal instability ratio is defined as the anisotropic energy divided by the thermal energy, kT, and is given by the formula KuV/kT, which should be greater than 50 in a conventional disk drive for adequate thermal stability. Ultimately, if a certain number of grains change their direction of magnetization due to thermal effects, the amplitude of a signal produced in the transducer head when the affected area is read will be decreased. Once the stored information decays beyond a threshold level, it will be impossible to properly read data written to the disk with the read head. In particular, the loss in the amplitude of a signal produced in the transducer head will cause data to be lost.
In order to address the effects of thermal decay, various measures have been taken. For example, error correction code may be used to restore data lost through processes such as thermal decay. However, the ability of error correction code to restore lost data is limited. In addition, the use of error correction code results in decreased user data density.
Attempts have also been made to produce magnetic disks having grains with large anisotropic energies. However, increasing the anisotropic energy of the grains generally requires larger grain sizes. As mentioned above, a larger grain size increases the noise of a signal produced by data stored on the magnetic disk. In particular, the transition noise is increased. Increased noise reduces the signal to noise ratio, and may adversely affect the bit error rate of the disk drive. In addition, if the anisotropic energy is increased by increasing the anisotropic constant (Ku), the coercivity (Hc) is also increased, and it becomes more difficult to write transitions to the magnetic disk.
Disk drive manufacturers have also limited the effects of thermal decay by requiring the magnetic material on a magnetic disk to be at least a certain minimum thickness, thereby increasing the volume of grains within a bit cell. However, as with increased grain sizes, increasing the thickness of the magnetic material increases noise when data is read from the magnetic disk.
For the above-stated reasons, it would be desirable to provide a method and an apparatus that allowed for increased data storage densities, without losing data due to thermal decay. In particular, it would be desirable to provide a method and an apparatus capable of providing an early warning of thermal decay. In addition, it would be advantageous to provide such a method and apparatus that are reliable in operation and that are inexpensive to implement.